1. Field of the Invention
The present invention relates to a fluorescent lamp. More particularly, it relates to a fluorescent lamp wherein penetration of mercury into the glass envelope is reduced or eliminated.
2. Description of Related Art
Mercury vapor discharge fluorescent lamps account for over 90 percent of commercial and office-space lighting. Fluorescent lamps typically include a glass envelope that is coated with a layer of phosphors to convert the ultraviolet radiation (UV) generated within the lamp into visible light.
Soda-lime glass is the most common type of glass for fluorescent lamps. Soda-lime glass is preferred because the sodium atoms (or ions) in the glass help prevent unconverted UV from escaping through the glass envelope.
However, a problem with soda-lime glass is that the sodium atoms in the glass attract mercury atoms from the mercury vapor within the lamp. This is because mercury and sodium form a stable amalgam which is retained in, thereby darkening, the glass envelope. This darkening can occur along the entire length of a fluorescent lamp, but often is most easily seen at the lamp ends, resulting in the end-discoloration or end-darkening commonly observed in fluorescent lamps.
As the glass envelope darkens, lumen maintenance of the fluorescent lamp is diminished because less visible light can escape. In addition, mercury atoms that have been absorbed into the glass envelope to become amalgamated with sodium are removed from the gaseous mercury phase within the lamp. The result is that the pressure of mercury vapor within the lamp is decreased over lamp life, and excess liquid mercury must be added to fluorescent lamps to make up the difference as mercury vapor absorbs into the glass envelope.
There is a need in the art for a fluorescent lamp that substantially reduces or prevents mercury vapor from absorbing into the glass envelope of the lamp. Preferably, such a lamp will have improved lumen maintenance and less discoloration of the glass envelope over existing fluorescent lamps.